Reversible resistive memory using polysilicon diodes as program selectors

ABSTRACT

Embodiments of reversible resistive memory cells using polysilicon diodes are disclosed. The programmable resistive devices can be fabricated using standard CMOS logic processes to reduce cell size and cost. In one embodiment, polysilicon diodes can be used as program selectors for reversible resistive memory cells that can be programmed based on magnitude, duration, voltage-limit, or current-limit of a supply voltage or current. These cells are PCRAM, RRAM, CBRAM, or other memory cells that have a reversible resistive element coupled to a polysilicon diode. The polysilicon diode can be constructed by P+/N+ implants on a polysilicon substrate as a program selector. The memory cells can be used to construct a two-dimensional memory array with the N-terminals of the diodes in a row connected as a wordline and the reversible resistive elements in a column connected as a bitline. By applying a voltage or a current to a selected bitline and to a selected wordline to turn on the diode, a selected cell can be programmed into different states reversibly based on magnitude, duration, voltage-limit, or current-limit. The data in the reversible resistive memory can also be read by turning on a selected wordline to couple a selected bitline to a sense amplifier. The wordlines may have high-resistivity local wordlines coupled to low-resistivity global wordlines through conductive contact(s) or via(s).

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims priority benefit of U.S. Provisional PatentApplication No. 61/375,653, filed on Aug. 20, 2010 and entitled “Circuitand System of Using Junction Diode As Program Selector for ResistiveDevices in CMOS Logic Processes,” which is hereby incorporated herein byreference; and U.S. Provisional Patent Application No. 61/375,660, filedon Aug. 20, 2010 and entitled “Circuit and System of Using PolysiliconDiode As Program Selector for Resistive Devices in CMOS LogicProcesses,” which is hereby incorporated herein by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to programmable memory devices, such asprogrammable resistive devices for use in memory arrays.

2. Description of the Related Art

A programmable resistive device is generally referred to a device'sresistance states that may change after means of programming. Resistancestates can also be determined by resistance values. For example, aresistive device can be a One-Time Programmable (OTP) device, such aselectrical fuse, and the programming means can apply a high voltage toinduce a high current to flow through the OTP element, such as fuse.When a high current flows through an OTP element by turning on a programselector, the OTP element can be programmed, or burned into a high orlow resistance state (depending on either fuse or anti-fuse).

An electrical fuse is a common OTP which is a programmable resistivedevice that can be constructed from a segment of interconnect, such aspolysilicon, silicided polysilicon, silicide, metal, metal alloy, orsome combination thereof. The metal can be aluminum, copper, or othertransition metals. One of the most commonly used electrical fuses is aCMOS gate, fabricated in silicided polysilicon, used as interconnect.The electrical fuse can also be one or more contacts or vias instead ofa segment of interconnect. A high current may blow the contact(s) orvia(s) into a very high resistance state. The electrical fuse can be ananti-fuse, where a high voltage makes the resistance lower, instead ofhigher. The anti-fuse can consist of one or more contacts or vias withan insulator in between. The anti-fuse can also be a CMOS gate coupledto a CMOS body with a thin gate oxide as insulator.

The programmable resistive device can be a reversible resistive devicethat can be programmed into a digital logic value “0” or “1”repetitively and reversibly. The programmable resistive device can befabricated from phase change material, such as Germanium (Ge), Antimony(Sb), and Tellurium (Te) with composition Ge₂Sb₂Te₅, (GST-225) orGeSbTe-like materials including compositions of Indium (In), Tin (Sn),or Selenium (Se). The phase change material can be programmed into ahigh resistance amorphous state or a low resistance crystalline state byapplying a short and high voltage pulse or a long and low voltage pulse,respectively. The reversible resistive device can be a Resistive RAM(RRAM) with cells fabricated from metal oxides between electrodes, suchas Pt/NiO/Pt, TiN/TiOx/HfO2/TiN, TiN/ZnO/Pt. The resistance states canbe changed reversibly and determined by polarity, magnitude, duration,or voltage/current-limit of pulse(s) to generate or annihilateconductive filaments. Another programmable resistive device similar toRRAM is a Conductive Bridge RAM (CBRAM) that is based onelectro-chemical deposition and removal of metal ions in a thinsolid-state electrolyte film. The electrodes can be oxidizable anode andan inert cathode and the electrolyte can be Ag- or Cu-doped chalcogenideglass such as GeSe or GeS, etc. The resistance states can be changedreversibly and determined by polarity, magnitude, duration, orvoltage/current-limit of pulse(s) to generate or annihilate conductivebridges. The programmable resistive device can be an MRAM (Magnetic RAM)with cells fabricated from magnetic multi-layer stacks that construct aMagnetic Tunnel Junction (MTJ). In a Spin Transfer Torque MRAM(STT-MRAM) the direction of currents applied to a MTJ determinesparallel or anti-parallel states, and hence low or high resistancestates.

A conventional programmable resistive memory cell is shown in FIG. 1.The cell 10 consists of a resistive element 11 and an NMOS programselector 12. The resistive element 11 is coupled to the drain of theNMOS 12 at one end, and to a positive voltage V+ at the other end. Thegate of the NMOS 12 is coupled to a select signal (Sel), and the sourceis coupled to a negative voltage V−. When a high voltage is applied toV+ and a low voltage to V−, the resistive device 10 can be programmed byraising the select signal (Sel) to turn on the NMOS 12. One of the mostcommon resistive elements is a silicided polysilicon, the same materialand fabricated at the same time as a MOS gate. The size of the NMOS 12,as program selector, needs to be large enough to deliver the requiredprogram current for a few microseconds. The program current for asilicided polysilicon is normally between a few milliamps for a fusewith width of 40 nm to about 20 mA for a fuse with width about 0.6 um.As a result, the cell size of an electrical fuse using silicidedpolysilicon tends to be very large.

Another conventional programmable resistive device 20 for Phase ChangeMemory (PCM) is shown in FIG. 2( a). The PCM cell 20 has a phase changefilm 21 and a bipolar transistor 22 as program selector with P+ emitter23, N-base 27, and P-sub collector 25. The phase change film 21 iscoupled to the emitter 23 of the bipolar transistor 22 in a ring shape,and to a positive voltage V+ at the other. The N-type base 27 of bipolartransistor 22 is coupled to a negative voltage V−. The collector 25 iscoupled to ground. By applying a proper voltage between V+ and V− for aproper duration of time, the phase change film 21 can be programmed intohigh or low resistance states, depending on voltage and duration.Conventionally, to program a phase-change memory to a high resistancestate (or reset state) requires about 3V for 50 ns and consumes about300 uA of current, or to program a phase-change memory to a lowresistance state (or set state) requires about 2V for 300 ns andconsumes about 100 uA of current.

FIG. 2( b) shows a cross section of a conventional bipolar transistor22. The bipolar transistor 22 includes a P+ active region 23, a shallowN well 24, an N+ active region 27, a P-type substrate 25, and a ShallowTrench Isolation (STI) 26 for device isolation. The P+ active region 23and N+ active region 27 couple to the N well 24 are the P and Nterminals of the emitter-base diode of the bipolar transistor 22, whilethe P− substrate 25 is the collector of the bipolar transistor 22. Thiscell configuration requires an N well 24 be shallower than the STI 26 toproperly isolate cells from each other and needs 3-4 more maskingoperations over the standard CMOS logic processes which makes it morecostly to fabricate.

Another programmable resistive device 20′ for Phase Change Memory (PCM)is shown in FIG. 2( c). The PCM cell 20′ has a phase change film 21′ anda diode 22′. The phase change film 21′ is coupled between an anode ofthe diode 22′ and a positive voltage V+. A cathode of the diode 22′ iscoupled to a negative voltage V−. By applying a proper voltage betweenV+ and V− for a proper duration of time, the phase change film 21′ canbe programmed into high or low resistance states, depending on voltageand duration. As an example of use of a diode as program selector foreach PCM cell as shown in FIG. 2( c), see Kwang-Jin Lee et al., “A 90 nm1.8V 512 Mb Diode-Switch PRAM with 266 MB/s Read Throughput,”International Solid-State Circuit Conference, 2007, pp. 472-273. Thoughthis technology can reduce the PCM cell size to only 6.8 F2 (F standsfor feature size), the diode requires very complicated process steps,such as Selective Epitaxial Growth (SEG), to fabricate, which would bevery costly for embedded PCM applications.

FIGS. 3( a) and 3(b) show several embodiments of an electrical fuseelement 80 and 84, respectively, fabricated from an interconnect. Theinterconnect serves as a particular type of resistive element. Theresistive element has three parts: anode, cathode, and body. The anodeand cathode provide contacts for the resistive element to be connectedto other parts of circuits so that a current can flow from the anode tocathode through the body. The body width determines the current densityand hence the electro-migration threshold for a program current. FIG. 3(a) shows a conventional electrical fuse element 80 with an anode 81, acathode 82, and a body 83. This embodiment has a large symmetrical anodeand cathode. FIG. 3( b) shows another conventional electrical fuseelement 84 with an anode 85, a cathode 86, and a body 87. Thisembodiment has an asymmetrical shape with a large anode and a smallcathode to enhance the electro-migration effect based on polarity andreservoir effects. The polarity effect means that the electro-migrationalways starts from the cathode. The reservoir effect means that asmaller cathode makes electro-migration easier because the smaller areahas lesser ions to replenish voids when the electro-migration occurs.The fuse elements 80, 84 in FIGS. 3( a) and 3(b) are relatively largestructures which makes them unsuitable for some applications.

FIGS. 4( a) and 4(b) show programming a conventional MRAM cell 210 intoparallel (or state 0) and anti-parallel (or state 1) by currentdirections. The MRAM cell 210 consists of a Magnetic Tunnel Junction(MTJ) 211 and an NMOS program selector 218. The MTJ 211 has multiplelayers of ferromagnetic or anti-ferromagnetic stacks with metal oxide,such as Al₂O₃ or MgO, as an insulator in between. The MTJ 211 includes afree layer stack 212 on top and a fixed layer stack 213 underneath. Byapplying a proper current to the MTJ 211 with the program selector CMOS218 turned on, the free layer stack 212 can be aligned into parallel oranti-parallel to the fixed layer stack 213 depending on the currentflowing into or out of the fixed layer stack 213, respectively. Thus,the magnetic states can be programmed and the resultant states can bedetermined by resistance values, lower resistance for parallel andhigher resistance for anti-parallel states. The resistances in state 0or 1 are about 5KΩ or 10KΩ, respectively, and the program currents areabout +/−100-200 μA. One example of programming an MRAM cell isdescribed in T. Kawahara, “2 Mb Spin-Transfer Torque RAM with Bit-by-BitBidirectional Current Write and Parallelizing-Direction Current Read,”International Solid-State Circuit Conference, 2007, pp. 480-481.

A diode can also be fabricated from polysilicon. FIG. 5( a) shows across section of a polysilicon diode. To form the polysilicon diode, thepolysilicon is implanted by N+ at one end and P+ at the other end with aspacing Lc in between that has intrinsic doping level. The intrinsicdoping level only means not intentionally doping any dopants but can beslightly N-type or P-type due to out diffusion or contamination. Asilicide block layer is applied to block silicide formation on thesurface of the polysilicon to thus prevent a short circuit. The two endsof P+ and N+ in polysilicon are further brought out as P and N terminalsof a diode with contacts. As an example of a polysilicon diode seeMing-Dou Ker et al., “Ultra High-Voltage Charge Pump Circuit inLow-Voltage Bulk CMOS Processes with Polysilicon Diodes,” IEEETransaction of Circuit and System-II, Vol. 54, No. 1, January 2007, pp.47-51.

FIG. 5( b) shows current verses voltage characteristics of a polysilicondiode, such as shown in FIG. 5( a). The current verses voltage curvesshow useful diode behavior such as a threshold voltage of about 0.6V anda leakage current of less than 1 nA. By varying the spacing Lc, thebreakdown voltage and leakage current for the polysilicon diode can beadjusted accordingly.

SUMMARY OF THE INVENTION

Embodiments of reversible resistive memory cells using polysilicondiodes are disclosed. The programmable resistive devices can befabricated using standard CMOS logic processes to reduce cell size andcost. In one embodiment, polysilicon diodes can be used as programselectors for reversible resistive memory cells that can be programmedbased on magnitude, duration, voltage-limit, or current-limit of asupply voltage or current. These cells are PCRAM, RRAM, CBRAM, or othermemory cells that have a reversible resistive element coupled to apolysilicon diode. The polysilicon diode can be constructed by P+/N+implants on a polysilicon substrate as a program selector. The memorycells can be used to construct a two-dimensional memory array with theN-terminals of the diodes in a row connected as a wordline and thereversible resistive elements in a column connected as a bitline. Byapplying a voltage or a current to a selected bitline and to a selectedwordline to turn on the diode, a selected cell can be programmed intodifferent states reversibly based on magnitude, duration, voltage-limit,or current-limit. The data in the reversible resistive memory can alsobe read by turning on a selected wordline to couple a selected bitlineto a sense amplifier. The wordlines may have high-resistivity localwordlines coupled to low-resistivity global wordlines through conductivecontact(s) or via(s).

The invention can be implemented in numerous ways, including as amethod, system, device, or apparatus (including graphical user interfaceand computer readable medium). Several embodiments of the invention arediscussed below.

As a reversible resistive memory, one embodiment of the invention can,for example, include at least a plurality of reversible resistive cells.Each of the reversible resistive cells can include a reversibleresistive film and a diode. The diode can serve as a program selectorand can be constructed from a polysilicon structure having a firstterminal from a first region with a first type of dopant, and a secondterminal from a second region with a second type of dopant. The firstterminal of the diode coupled to a first terminal of the reversibleresistive film. The reversible resistive film being programmablereversibly by conducting a current flowing through the reversibleresistive film. The current can be high or short duration forprogramming one state and low or long duration for programming anotherstate. The embodiment of the reversible resistive memory can alsoinclude: a plurality of local wordlines, each coupled to a plurality ofthe reversible resistive cells via the second terminal of the diodes andhaving a first resistivity; a plurality of global wordlines, eachcoupled to at least one of the local wordlines and having a secondresistivity; and a plurality of bitlines, each coupled to a plurality ofthe reversible resistive cells via the second terminal of reversibleresistive film.

As an electronic system, one embodiment of the invention can, forexample, include at least a processor, and a reversible resistive memoryoperatively connected to the processor. The reversible resistive memorycan include at least a plurality of reversible resistive cells. Each ofthe reversible resistive cells can include a reversible resistive filmand a diode. The diode can serve as a program selector and can beconstructed from a polysilicon structure having a first terminal from afirst region with a first type of dopant and a second terminal from asecond region with a second type of dopant, with the first terminal ofthe diode coupled to a first terminal of the reversible resistive film.The reversible resistive film can be programmable reversibly byconducting a current flowing through the reversible resistive film,where the current is high or short duration for programming one stateand low or long duration for programming another state. The reversibleresistive memory can also include a plurality of local wordlines, eachcoupled to a plurality of the reversible resistive cells via the secondterminal of the diodes and having a first resistivity; a plurality ofglobal wordlines, each coupled to at least one of the local wordlinesand having a second resistivity; and a plurality of bitlines, eachcoupled to a plurality of the reversible resistive cells via the secondterminal of reversible resistive film.

As a method for programming a reversible resistive memory, oneembodiment of the method can, for example, include: providing aplurality of reversible resistive cells, each of the reversibleresistive memory cells including a reversible resistive film and adiode, the diode serves as program selector and is constructed from apolysilicon structure having a first terminal from a first region with afirst type of dopant and a second terminal from a second region with asecond type of dopant, the first terminal of the diode being coupled toa first terminal of the reversible resistive film, the reversibleresistive film being programmable reversibly by conducting a currentflowing through the reversible resistive film; providing a plurality oflocal wordlines, each coupled to a plurality of the reversible resistivecells via the second terminal of the diodes and having a firstresistivity; providing a plurality of global wordlines, each coupled toat least one of the local wordlines and having a second resistivity;providing a plurality of bitlines, each coupled to a plurality of thereversible resistive cells via a second terminal of reversible resistivefilm; and programming at least one of the reversible resistive cells byapplying voltages to a selected one of the global wordlines and to aselected one of the bitlines, and generating a high or short-durationcurrent to reversibly program into one state and a low or long-durationcurrent to reversibly program into another state.

Other aspects and advantages of the invention will become apparent fromthe following detailed description taken in conjunction with theaccompanying drawings which illustrate, by way of example, theprinciples of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will be readily understood by the followingdetailed descriptions in conjunction with the accompanying drawings,wherein like reference numerals designate like structural elements, andin which:

FIG. 1 shows a conventional programmable resistive memory cell.

FIG. 2( a) shows another conventional programmable resistive device forPhase Change Memory (PCM) using a bipolar transistor as programselector.

FIG. 2( b) shows a cross section of a conventional Phase Change Memory(PCM) using a bipolar transistor as program selector.

FIG. 2( c) shows another conventional Phase Change Memory (PCM) cellusing a diode as program selector.

FIGS. 3( a) and 3(b) show several embodiments of an electrical fuseelement, respectively, fabricated from an interconnect.

FIGS. 4( a) and 4(b) show programming a conventional MRAM cell intoparallel (or state 0) and anti-parallel (or state 1) by currentdirections.

FIG. 5( a) shows a cross section of a polysilicon diode.

FIG. 5( b) shows current verses voltage characteristics of a polysilicondiode, such as shown in FIG. 5( a).

FIG. 6( a) shows a block diagram of a memory cell using a polysilicondiode as program selector according to one embodiment.

FIG. 6( b) shows a top view of a fuse cell using a polysilicon diode asprogram selector according to one embodiment.

FIG. 7( a) shows an electrical fuse element according to one embodiment.

FIGS. 7( b), (c), and (d) show a top view of electrical fuse cells usingpolysilicon as fuse elements and polysilicon diodes according to variousembodiments.

FIG. 8( a) shows a top view of a polysilicon electrical fuse cell havinga common P terminal as anode shared by 4 polysilicon diodes with fourfuse elements in accordance with one embodiment.

FIG. 8( b) shows a top view of a 4×3 array of via1 fuses built onpolysilicon in accordance with one embodiment.

FIG. 8( c) shows a top view of an array of polysilicon fuses with adiode constructed from a P+ polysilicon and an N-type buried layer atcross points according to one embodiment.

FIG. 9( a) shows a cross section of a programmable resistive device cellusing phase-change material as a resistive element, with buffer metalsand a polysilicon diode, according to one embodiment.

FIG. 9( b) shows a top view of a PCM cell with a polysilicon diode asprogram selector having a cell boundary in accordance with oneembodiment.

FIG. 10 shows one embodiment of an MRAM cell using diodes as programselectors in accordance with one embodiment.

FIG. 11( a) shows a top view of an MRAM cell with an MTJ as a resistiveelement and with polysilicon diodes and as program selectors inaccordance with one embodiment.

FIG. 11( b) shows another top view of a MRAM cell with the MTJ as aresistive element and with the polysilicon diodes as program selectorsin accordance with another embodiment.

FIG. 11( c) shows another top view of a MRAM cell by using onepolysilicon diode and one junction diode in accordance with oneembodiment.

FIG. 11( d) shows another top view of a MRAM cell by using onepolysilicon diode and one junction diode with an abutted contact inaccordance with another embodiment.

FIG. 12( a) shows one embodiment of a three-terminal 2×2 MRAM cell arrayusing diodes as program selectors and the condition to program theupper-right cell into 1 in accordance with one embodiment.

FIG. 12( b) shows alternative conditions to program the upper-right cellinto 1 in a 2×2 MRAM array in accordance with one embodiment.

FIG. 13( a) shows one embodiment of a three-terminal 2×2 MRAM cell arrayusing diodes as program selectors and the condition to program theupper-right cell into 0 in accordance with one embodiment.

FIG. 13( b) shows alternative conditions to program the upper-right cellinto 0 in a 2×2 MRAM array in accordance with one embodiment.

FIGS. 14( a) and 14(b) show one embodiment of programming 1 and 0 intothe upper-right cell, respectively, in a two-terminal 2×2 MRAM cellarray in accordance with one embodiment.

FIG. 15 shows a portion of a programmable resistive memory constructedby an array of n-row by (m+1)-column non-MRAM cells and n wordlinedrivers in accordance with one embodiment.

FIG. 16( a) shows a portion of a programmable resistive memoryconstructed by an array of 3-terminal MRAM cells according to oneembodiment.

FIG. 16( b) shows another embodiment of constructing a portion of MRAMmemory with 2-terminal MRAM cells.

FIGS. 17( a), 17(b), and 17(c) show three other embodiments ofconstructing reference cells for differential sensing.

FIG. 18( a) shows a schematic of a wordline driver circuit according toone embodiment.

FIG. 18( b) shows a schematic of a bitline circuit according to oneembodiment.

FIG. 18( c) shows a portion of memory with an internal power supply VDDPcoupled to an external supply VDDPP and a core logic supply VDD throughpower selectors.

FIG. 19( a) shows one embodiment of a schematic of a pre-amplifieraccording to one embodiment.

FIG. 19( b) shows one embodiment of a schematic of an amplifieraccording to one embodiment.

FIG. 19( c) shows a timing diagram of the pre-amplifier and theamplifier in FIGS. 19( a) and 19(b), respectively.

FIG. 20( a) shows another embodiment of a pre-amplifier, similar to thepre-amplifier in FIG. 18( a).

FIG. 20( b) shows level shifters according to one embodiment.

FIG. 20( c) shows another embodiment of an amplifier with current-mirrorloads.

FIG. 21( a) depicts a method of programming a programmable resistivememory in a flow chart according to one embodiment.

FIG. 21( b) depicts a method of reading a programmable resistive memoryin a flow chart according to one embodiment.

FIG. 22 shows a processor system according to one embodiment.

DETAILED DESCRIPTION OF EMBODIMENTS OF THE INVENTION

Embodiments disclosed herein use a polysilicon diode as program selectorfor a programmable resistive device. The diode can comprise P+ and N+implants on a polysilicon substrate. Since the P+ and N+ implants andpolysilicon are readily available in standard CMOS logic processes,these devices can be formed in an efficient and cost effective manner.There are no additional masks or process steps to save costs. Theprogrammable resistive device can also be included within an electronicsystem.

FIG. 6( a) shows a block diagram of a memory cell 30 using a polysilicondiode according to one embodiment. In particular, the memory cell 30includes a resistive element 31 and a polysilicon diode 32. Theresistive element 31 can be coupled between an anode of the polysilicondiode 32 and a positive voltage V+. A cathode of the polysilicon diode32 can be coupled to a negative voltage V−. In one implementation, thememory cell 30 can be a fuse cell with the resistive element 31operating as an electrical fuse. The polysilicon diode 32 can serve as aprogram selector. The coupling of the resistive element 31 and thepolysilicon diode 32 between the supply voltages V+ and V− can beinterchanged. By applying a proper voltage between V+ and V− for aproper duration of time, the resistive element 31 can be programmed intohigh or low resistance states, depending on voltage and duration,thereby programming the memory cell 30 to store a data value (e.g., bitof data).

FIG. 6( b) shows a top view of a fuse cell 30 using a polysilicon diodeas program selector according to one embodiment. The fuse cell 30include an electrical fuse element 31 coupled to a first supply voltageV+ and to a diode 32. The diode 32 serves as the program selector forthe fuse cell 30. The diode 32 is built on a piece of polysilicon 34,i.e., polysilicon substrate. The P+ and N+ implants 33 and 37 are usedto create source or drain of a PMOS or NMOS device, respectively, andthus form the P and N terminals of the diode 32 on the polysilicon 34. ASilicide Block Layer (SBL) 36 blocks silicide formation on the top ofpolysilicon surface to prevent P and N terminals of the diode 32 fromshorting. A space d between P+ 33 and N+ 37 can be used to adjustbreakdown voltage and leakage current. A layer 39 is optional tointroduce NLDD, PLDD, NMOS Vt, PMOS Vt or others implants over theintrinsic dopant level into the area between N+ and P+ implant layers 37and 33 to further control the diode's turn-on resistance. The areaimplanted by the optional layers 39 can be generated from standard CMOSmasks by operations on layer database without any additional costs.

FIG. 7( a) shows an electrical fuse element 88 according to oneembodiment. The electrical fuse element 88 can, for example, by used asthe electrical fuse element 31 illustrated in FIGS. 6( a) and 6(b). Theelectrical fuse element 88 includes an anode 89, a cathode 90, and abody 91. In this embodiment, the electrical fuse element 88 is a barshape with a small anode 89 and cathode 90 to reduce area. The anode 89and cathode 90 may protrude from the body 91 to make contacts. Thecontact number can be one (1) for both the anode 89 and the cathode 90so that the area can be very small. However, the contact area for anode89 is often made larger so that the anode 89 can resistelectro-migration more than the cathode 90. The fuse body 91 can haveabout 1-5 squares, namely, the length to width ratio is about 1-to-5, tooptimize cell area and program current. The fuse element 88 has a P+implant 92 covering half of the body 91 and the cathode 90, while an N+implant over the rest of area. This embodiment makes the fuse element 88behaves like a reverse biased diode to increase resistance after beingprogrammed, when the silicide on top is depleted by electro-migration,ion diffusion, silicide decomposition, and other effects.

The above scheme can be realized for those fuse elements consisting ofpolysilicon, silicided polysilicon, or other CMOS gate material so thatP+ and N+ implants can create a diode. For example, if a metal-gate CMOShas a sandwich structure of polysilicon between metal alloy layers, themetal alloy layers may be blocked by masks generated from the layoutdatabase to create a diode on the fuse element.

FIGS. 7( b), 7(c), and 7(d) show a top view of electrical fuse cellsusing polysilicon as fuse elements and polysilicon diodes according tovarious embodiments. Since the fuse element is fabricated from apolysilicon and the diode is built on a polysilicon substrate, the fuseelement and the diode can be integrally formed. By integrally formingthe fuse element and the diode, programmable resistive cells that arecompact can be formed because redundant contacts and spacing can beavoided.

FIG. 7( b) shows a one-piece polysilicon electrical fuse cell 80 havinga fuse element with anode 89, body 87, and a polysilicon diode 81according to one embodiment. The anode 89, the body 87 and a P terminalof the polysilicon diode 81 are covered by a P+ implant 83, while thecathode 88 (the N terminal of the polysilicon diode 81) is covered by anN+ implant 84. Silicide Block Layer 82 can be provided over at leastadjacent portions of the P+ implant 83 and the N+ implant 84 to blocksilicide grown on the top of polysilicon to prevent N and P terminalsfrom shorting. The anode 89 is coupled to V+ and the cathode 88 iscoupled to V−. When a high voltage is applied to V+ and a low voltage toV−, a current flows from the anode 89 through body 87 to the P terminalof the polysilicon diode 81 and then to the N terminal 88 of thepolysilicon diode 81. In this embodiment, the body 87 of fuse elementhas a P+ implant 83.

FIG. 7( c) shows another one-piece polysilicon electrical fuse cell 90having a diode 91 and a fuse element with a body 97 and a cathode 99according to one embodiment. The P terminal of the diode 91 is coupledto a supply voltage V+. The N terminal of the diode 91 is coupled to thebody 97 of the fuse element, which is then coupled to another supplyvoltage V− through the cathode 99. The P terminal 98 of the diode 91,lower half of the body 97, and the cathode 99 are covered by a P+implant 93, while the N terminal of the diode 91 and the upper portion(e.g., half) of the body 97 are covered by an N+ implant 94. A silicideBlock Layer 92 can block silicide grown on top of polysilicon to preventthe N and P terminals from shorting. When a high voltage is applied toV+ and a low voltage to V−, a current flows from the P terminal 98 ofthe polysilicon diode 91 through the N terminal of the polysilicon diode91 and the body 97 to the cathode 99. In this embodiment, the body 97has N+/P+ implants covering respective portions (e.g., approximatelyhalf N+ and half P+). When the silicide on the top of the polysilicondiode 91 is depleted, the body 97 behaves like a reverse biased diode toincrease the post-program resistance. Besides, the cathode 99 has asmaller area than the anode, i.e., the P terminal 98 of the polysilicondiode 91, which serves to enhance electro-migration.

FIG. 7( d) shows a top view of a pair of staggered one-piece electricalfuse cells according to one embodiment. Like the polysilicon electricalfuse cell 90, each of the electrical fuse cells in the pair are formedof a polysilicon resistive element (e.g., fuse element) and apolysilicon diode. The second electrical fuse cell is rotated180-degrees and placed adjacent the first electrical fuse cell so thatthe large anode and small cathode are adjacent next to each other for acompact (or dense) arrangement that consumed a relatively small area.

If the program current is not very high, the size of the one-pieceelectrical fuse cells can be further reduced. FIG. 8( a) shows a topview of a polysilicon electrical fuse cell 91 having a common P terminal98 (implanted by P+ 93) as anode shared by 4 polysilicon diodes 96 foruse with four fuse elements 95 in accordance with one embodiment. TheSBL 92 blocks silicide formation on the top of the polysilicon diode 96to separate P terminal 98 and N terminal of diodes 96 for eachelectrical fuse cell. A polysilicon diode 96 which serves as programselector is coupled to a portion of polysilicon covered by an N+ 94 asthe N terminal, which is further coupled to a body 97 and to a cathode99. The P terminal 98 is coupled to a high voltage supply V+ and eachcathode 99 is coupled to a low voltage supply V−. By applying propervoltages to V+ and V−, a current will flow to change the resistance ofthe body 97 accordingly.

FIG. 8( b) shows a top view of a 4×3 array of via1 fuses 90 built onpolysilicon in accordance with one embodiment. The array of via1 fuses90 has four rows of polysilicon diode 91, each has alternative N+contacts 97 and P+ contacts 99 covered by N+ implant 94 and P+ implant93, respectively, with metal1 on top. The P+ contacts 99 and N+ contacts97, which are separated by a SBL 92 act as the P and N terminals ofdiodes. The P+ contacts 99 in the same columns are connected by a metal2running vertically as bitlines. Similarly, the N+ contacts 97 in thesame rows are connected by a metal3 running horizontally as wordlines. Aparticular programmable resistive cell 96 can be programmed by applyinga high voltage to a selected bitline and a low voltage or ground to aselected wordline to conduct a current flowing from metal2 bitline,via1, metal1, contact, polysilicon, through the selected diode andmetal3 to ground. If the metal1, metal2, and metal3 have higherthreshold to blow up, either via1 (connect between metal1 and metal2) orcontact (connect between metal1 and polysilicon) will be programmed. Toensure via1 be programmed, the metals are preferably made wider and thenumbers of via2 and contacts are preferably more than one. In oneembodiment, the via1 can be built on the top of a P+ contact 99 to makea unit cell very small. Those skilled in the art understand that inother embodiments the via1 fuses can be extended to contact, via2,various other types of contact/via fuses or metal schemes may be used,and the numbers of rows and column may vary, the rows and columns areinterchangeable.

FIG. 8( c) shows a top view 60 of a 4×3 array of polysilicon fuses witha diode constructed from a P+ polysilicon 61 and an N-type buried layer62 at cross points according to one embodiment. The buried layer 62 isan active region with an implant before the N+ or P+ implants 64 and 63for sources or drains of CMOS. Therefore, the buried layer 62 can beused as an interconnect underneath and can be crossed by the P+polysilicon 61 provided above. In this embodiment, the buried layer 62is implanted with an N-type dopant after active-region isolation isfabricated. The gate oxide grown on top of the buried layer 62 isstripped before the P+ polysilicon 61 is deposited. The polysilicon ispartly implanted by a P-type dopant as the P terminal of a diode, andpartly implanted by an N-type dopant as an N-type polysilicon fuse,though a silicide on top connects the two parts. As a result, a verycompact P/N polysilicon fuse with a P-type polysilicon and N-type bulksilicon as two terminals of a diode is constructed. Metal 2 straps theburied layer 62, (not shown in FIG. 8( c)), running in the verticaldirection as a bitline, and metal 3 straps the polysilicon throughcontacts 65 running in the horizontal direction as a wordline.

FIG. 9( a) shows a cross section of a programmable resistive device cell50 using phase-change material as a resistive element 42, with buffermetals 41 and 43, and a polysilicon diode 30 on a silicon substrate witha dielectric 49, according to one embodiment. The polysilicon diode 30has a P+ implant 33 and N+ implant 37 on a polysilicon substrate 31 as Pand N terminals through anode contact 32 and cathode contact 39. A SBL35 separates the P+ implant 33 from the N+ implant 37. The anode 32 ofthe polysilicon diode 30 is coupled to a lower metal 41 as a bufferlayer through a contact plug 40-1. The lower metal 41 is then coupled toa thin film of phase-change material 42 (e.g., PCM film) and an uppermetal buffer layer 43 through a contact plug 40-2. The upper metal iscoupled to another metal 44 to act as a bitline (BL) through a plug40-3. The PCM film 42 can have a chemical composition of Germanium (Ge),Antimony (Sb), and/or Tellurium (Te), such as Ge_(x)Sb_(y)Te_(z) (x, y,and z are any arbitrary numbers) or, more particularly, Ge₂Sb₂Te₅.(GST-225). The PCM film 42 can be doped with at least one or more ofIndium (In), Tin (Sn), or Selenium (Se) to enhance performance. The PCMstructure can be substantially planar, which means the PCM film area islarger than the film contact area coupled to the program selector(polysilicon diode 30), or the height from the surface of the siliconsubstrate to the film is much smaller than the dimensions of the filmparallel to silicon substrate. In this embodiment, the active area ofPCM film 42 (e.g., GST film) is much larger than the contact area sothat the programming characteristics can be more uniform andreproducible. The GST film 42 is not a vertical structure and does notsit on top of a tall contact, which is more suitable for embedded PCMapplications, especially when the polysilicon diode 30 is used asprogram selector to make the cell size very small. For those skilled inthe art understand that the structure and fabrication processes may varyand that the structures of GST film and buffer metals described aboveare for illustrative purpose.

FIG. 9( b) shows a top view of a PCM cell using a polysilicon diode asprogram selector with a cell boundary 70 in accordance with oneembodiment. The PCM cell has a polysilicon diode 71 and a phase-changematerial 75. The polysilicon diode 71 has an anode 72 and a cathode 79covered by P+ and N+ implants 73 and 77, respectively. A SBL 80 blockssilicide formation on the top of polysilicon diode 71 to prevent theanode 72 from shorting to the cathode 79. The anode 72 is coupled to thephase-change film 75 through a metal1 76. The phase-change film 75 isfurther coupled to a metal3 bitline (BL) 78 running vertically. Thecathode 79 of the polysilicon diode 71 is connected by a metal2 wordline(WL) 77 running horizontally. By applying a proper voltage between thebitline 78 and the wordline 77 for a suitable duration, the phase-changefilm can be programmed into a 0 or 1 state accordingly. Sinceprogramming the phase-change film is based on raising the temperaturerather than electro-migration as with an electrical fuse, thephase-change film (e.g., GST film) can be symmetrical in area for bothanode and cathode. Those skilled in the art understand that thephase-change film, structure, layout style, and metal schemes may varyand that the above description is for illustrative purpose.

Programming a phase-change memory, such as a phase-change film, dependson the physical properties of the phase-change film, such as glasstransition and melting temperatures. To reset, the phase-change filmneeds to be heated up beyond the melting temperature and then quenched.To set, the phase-change film needs to be heated up between melting andglass transition temperatures and then annealed. A typical phase-changefilm has glass transition temperature of about 200° C. and meltingtemperature of about 600° C. These temperatures determine the operationtemperature of a phase-change memory because the resistance state maychange after staying in a particular temperature for a long time.However, most applications require retaining data for 10 years for theoperation temperature from 0 to 85° C. or even from −40 to 125° C. Tomaintain cell stability over the device's lifetime and over such a widetemperature range, periodic reading and then writing back data into thesame cells can be performed. The refresh period can be quite long, e.g.,longer than a second, such as days, weeks, or even months. The refreshmechanism can be generated inside the memory or triggered from outsidethe memory. The long refresh mechanism can serve to maintain cellstability be applied to other emerging memories such as RRAM, CBRAM, andMRAM, etc.

FIG. 10 shows one embodiment of an MRAM cell 310 using diodes 317 and318 as program selectors in accordance with one embodiment. The MRAMcell 310 in FIG. 10 is a three-terminal MRAM cell. The MRAM cell 310 hasan MTJ 311, including a free layer stack 312 and a fixed layer stack 313with a dielectric film in between, and the two diodes 317 and 318. Thefree layer stack 312 is coupled to a supply voltage V, and coupled tothe fixed layer stack 313 through a metal oxide such as Al₂O₃ or MgO.The diode 317 has the N terminal coupled to the fixed layer stack 313and the P terminal coupled to V+ for programming a 1. The diode 318 hasthe P terminal coupled to the fixed layer stack 313 and the N terminalcoupled to V− for programming a 0. If V+ voltage is higher than V, acurrent flows from V+ to V to program the MTJ 311 into state 1.Similarly, if V− voltage is lower than V, a current flows from V to V−to program the MTJ 311 into state 0. During programming, the other diodeis supposedly cutoff. For reading, V+ and V− can be both set to 0V andthe resistance between node V and V+/V− can be sensed to determinewhether the MTJ 311 is in state 0 or 1.

FIG. 11( a) shows a top view of an MRAM cell 80 with an MTJ 89 as aresistive element and with polysilicon diodes 86 and 88 as programselectors in accordance with one embodiment. The MTJ 89 has a slantellipse shape with a free layer stack on top, a fixed layer stackunderneath, and a dielectric in between to constitute a magnetictunneling junction. The MTJ 89 is coupled to a metal3 bitline on toprunning vertically. Program-1 diode 86 and program-0 diode 88 arepolysilicon diodes built on two sections (e.g., rectangles) ofpolysilicon 81 and are placed side by side and connected at one end,i.e., the N terminal of the diode 86 is connected to the P terminal ofthe diode 88. A P+ implant 83 and an N+ implant 87 define the P and Nterminals of the diodes 86 and 88. A SBL 82 can be provided to preventshorting. The program-1 diode 86 has the P terminal coupled to a supplyvoltage V+ and has the N terminal coupled to the fixed stack of MTJ 89.The program-0 diode 88 has the N terminal coupled to a supply voltage V−and has the P terminal coupled to the fixed stack of MTJ 89. The V+ andV− voltages of each MRAM cell 80 are connected as metal2 wordlines, WLPand WLN, running horizontally.

FIG. 11( b) shows another top view of a MRAM cell 80′ with the MTJ 89 asa resistive element and with the polysilicon diodes 86 and 88 as programselectors in accordance with another embodiment. The MTJ 89 has a slantellipse shape with a free layer stack on top, a fixed layer stackunderneath, and a dielectric in between to constitute a magnetictunneling junction. The MTJ 89 is coupled to a metal3 bitline on toprunning vertically. The Program-1 diode 86 and the program-0 diode 88are connected back to back in a one-piece polysilicon section 81 (e.g.,rectangle). Namely, the N terminal of the diode 86 is connected to the Pterminal of the diode 88. A P+ implant 83 and an N+ implant 87 definethe P and N terminals of the diodes 86 and 88. The SBL 82 has twosections provided to prevent shorting. The program-1 diode 86 has the Pterminal coupled to a supply voltage V+ and the N terminal coupled tothe fixed stack of MTJ 89. The program-0 diode 88 has the N terminalcoupled to a supply voltage V− and the P terminal coupled to the fixedstack of MTJ 89. The V+ and V− voltages of each MRAM cell 80′ areconnected in metal2 wordlines, WLP and WLN, running horizontally. Thetop views shown in FIGS. 11( a) and (b) are for illustrative purposesand those skilled in the art understand that there are many ways ofconstructing polysilicion diodes coupled with a MTJ and metal schemes.

FIG. 11( c) shows another top view of a MRAM cell 80″ by using onepolysilicon diode 88 and one junction diode 86′ in accordance with oneembodiment. The P+ implant 83′ and N+ implant 87 define the P and the Nterminals of diodes the 88 and 86′ over a polysilicon section 91 and anactive region 92, respectively. The junction diode 86′ is housed in an Nwell for CMOS devices, and has the P terminal coupled to a supplyvoltage V+ and the N terminal coupled to the P terminal of thepolysilicon diode 88 and then to another supply voltage V through theMTJ 89 and the metal1 93. A dummy CMOS gate 85 can separate the P and Nterminals of the junction diode 86. Similarly, the polysilicon diode 88has the N terminal coupled to a supply voltage V− and the P terminalcoupled to the N terminal of the junction diode 86′ and to a supplyvoltage V through the MTJ 89 and the metal1 93. The SBL 82 separates theP and N terminals of the polysilicon diode 88. The supply voltage V isfurther coupled to a metal3 bitline running vertically, while the supplyvoltages V+ and V− are coupled to metal2 wordlines, WLP and WLN, runninghorizontally.

FIG. 11( d) shows another top view of a MRAM cell 80″' by using onepolysilicon diode 88 and one junction diode 86′ with an abutted contact84 in accordance with another embodiment. The P+ implant 83′ and the N+implant 87 define the P and N terminals of the diodes 88 and 86′ over apolysilicon section 91 and an active region 92, respectively. Thejunction diode 86′ is housed in an N well for CMOS devices, and has theP terminal coupled to a supply voltage V+ and the N terminal coupled tothe P terminal of the polysilicon diode 88 and to another supply voltageV through the MTJ 89 and the metal1 93. The dummy CMOS gate 85 canseparate the P and N terminals of the junction diode 86′. Similarly, thepolysilicon diode 88 has the N terminal coupled to a supply voltage V−and the P terminal coupled to the N terminal of the junction diode 86′and to a supply voltage V through the MTJ 89 and the metal1 93. The SBL82 separates the P and N terminals of the polysilicon diode 88. Thesupply voltage V is further coupled to a metal3 bitline runningvertically, while the supply voltage V+ and V− are coupled to metal2wordlines, WLP and WLN, running horizontally. A contact to couple the Nterminal of the junction diode 86′ and the P terminal of the polysilicondiode 88 is through an abutted contact 84. The polysilicon 91 overlapsinto the action region 92 with a metal1 93 on top to connect polysiliconand active region in a single contact 84. Hence, two contacts are mergedinto one and thus polysilicon-to-active spacing can be saved to reducearea and costs, thereby rendering this embodiment particularlyefficient. Using of junction diode and polysilicon diodes in FIGS. 11(c) and 11(d) as program-1 and program-0 diodes, respectively, can beinterchangeable. Those skilled in the art understand that variousembodiments of mixing different kinds of diodes in variousconfigurations for memory are possible and that they are still withinthe scope of this invention.

FIG. 12( a) shows one embodiment of a three-terminal 2×2 MRAM cell arrayusing diodes 317 and 318 as program selectors and the condition toprogram 1 in a cell in accordance with one embodiment. Cells 310-00,310-01, 310-10, and 310-11 are organized as a two-dimensional array. Thecell 310-00 has a MTJ 311-00, a program-1 diode 317-00, and a program-0diode 318-00. The MTJ 311-00 is coupled to a supply voltage V at oneend, to the N terminal of the program-1 diode 317-00 and to the Pterminal of the program-0 diode 318-00 at the other end. The P terminalof the program-1 diode 317-00 is coupled to a supply voltage V+. The Nterminal of the program-0 diode 318-00 is coupled to another supplyvoltage V-. The other cells 310-01, 310-10, and 310-11 are similarlycoupled. The voltage Vs of the cells 310-00 and 310-10 in the samecolumns are connected to BL0. The voltage Vs of the cells 310-01 and310-11 in the same column are connected to BL1. The voltages V+ and V−of the cells 310-00 and 310-01 in the same row are connected to WL0P andWL0N, respectively. The voltages V+ and V− of the cells 310-10 and310-11 in the same row are connected to WL1P and WL1N, respectively. Toprogram a 1 into the cell 310-01, WL0P is set high and BL1 is set low,while setting the other BL and WLs at proper voltages as shown in FIG.12( a) to disable the other program-1 and program-0 diodes. The boldline in FIG. 12( a) shows the direction of current flow.

FIG. 12( b) shows alternative program-1 conditions for the cell 310-01in a 2×2 MRAM array in accordance with one embodiment. For example, toprogram a 1 into cell 310-01, set BL1 and WL0P to low and high,respectively. If BL0 is set to high in condition 1, the WL0N and WL1Ncan be either high or floating, and WL1P can be either low or floating.The high and low voltages of an MRAM in today's technologies are about2-3V for high voltage and 0 for low voltage, respectively. If BL0 isfloating in condition 2, WL0N and WL1N can be high, low, or floating,and WL1P can be either low or floating. In a practical implementation,the floating nodes are usually coupled to very weak devices to a fixedvoltage to prevent leakage. One embodiment of the program-1 condition isshown in FIG. 12( a) without any nodes floating.

FIG. 13( a) shows one embodiment of a three-terminal 2×2 MRAM cell arraywith MTJ 311 and diodes 317 and 318 as program selectors and thecondition to program 0 in a cell in accordance with one embodiment. Thecells 310-00, 310-01, 310-10, and 310-11 are organized as atwo-dimensional array. The cell 310-00 has a MTJ 311-00, a program-1diode 317-00, and a program-0 diode 318-00. The MTJ 311-00 is coupled toa supply voltage V at one end, to the N terminal of program-1 diode317-00 and to the P terminal of program-0 diode 318-00 at the other end.The P terminal of the program-1 diode 317-00 is coupled to a supplyvoltage V+. The N terminal of the program-0 diode 318-00 is coupled toanother supply voltage V−. The other cells 310-01, 310-10, and 310-11are similarly coupled. The voltage Vs of the cells 310-00 and 310-10 inthe same columns are connected to BL0. The voltage Vs of the cells310-01 and 310-11 in the same column are connected to BL1. The voltagesV+ and V− of the cells 310-00 and 310-01 in the same row are connectedto WL0P and WL0N, respectively. The voltages V+ and V− of the cells310-10 and 310-11 in the same row are connected to WL1P and WL1N,respectively. To program a 0 into the cell 310-01, WL0N is set low andBL1 is set high, while setting the other BL and WLs at proper voltagesas shown in FIG. 13( a) to disable the other program-1 and program-0diodes. The bold line in FIG. 13( a) shows the direction of currentflow.

FIG. 13( b) shows alternative program-0 conditions for the cell 310-01in a 2×2 MRAM array in accordance with one embodiment. For example, toprogram a 0 into cell 310-01, set BL1 and WL0N to high and low,respectively. If BL0 is set to low in condition 1, the WL0P and WL1P canbe either low or floating, and WL1N can be either high or floating. Thehigh and low voltages of an MRAM in today's technologies are about 2-3Vfor high voltage and 0 for low voltage, respectively. If BL0 is floatingin condition 2, WL0P and WL1P can be high, low, or floating, and WL1Ncan be either high or floating. In a practical implementation, thefloating nodes are usually coupled to very weak devices to a fixedvoltage to prevent leakage. One embodiment of the program-0 condition isas shown in FIG. 13( a) without any nodes floating.

The cells in 2×2 MRAM arrays in FIGS. 12( a), 12(b), 13(a) and 13(b) arethree-terminal cells, namely, cells with V, V+, and V− nodes. However,if the program voltage VDDP is less than twice a diode's thresholdvoltage Vd, i.e. VDDP<2*Vd, the V+ and V− nodes of the same cell can beconnected together as a two-terminal cell. Since Vd is about 0.6-0.7V atroom temperature, this two-terminal cell works if the program highvoltage is less than 1.2V and low voltage is 0V. This is a commonvoltage configuration of MRAM arrays for advanced CMOS technologies thathas supply voltage of about 1.0V. FIGS. 14( a) and 14(b) show schematicsfor programming a 1 and 0, respectively, in a two-terminal 2×2 MRAMarray.

FIGS. 14( a) and 14(b) show one embodiment of programming 1 and 0,respectively, in a two-terminal 2×2 MRAM cell array in accordance withone embodiment. The cells 310-00, 310-01, 310-10, and 310-11 areorganized in a two-dimensional array. The cell 310-00 has the MTJ311-00, the program-1 diode 317-00, and the program-0 diode 318-00. TheMTJ 311-00 is coupled to a supply voltage V at one end, to the Nterminal of program-1 diode 317-00 and the P terminal of program-0 diode318-00 at the other end. The P terminal of the program-1 diode 317-00 iscoupled to a supply voltage V+. The N terminal of the program-0 diode318-00 is coupled to another supply voltage V−. The voltages V+ and V−are connected together in the cell level if VDDP<2*Vd can be met. Theother cells 310-01, 310-10 and 310-11 are similarly coupled. Thevoltages Vs of the cells 310-00 and 310-10 in the same columns areconnected to BL0. The voltage Vs of the cells 310-01 and 310-11 in thesame column are connected to BL1. The voltages V+ and V− of the cells310-00 and 310-01 in the same row are connected to WL0. The voltages V+and V− of the cells 310-10 and 310-11 in the same row are connected toWL1.

To program a 1 into the cell 310-01, WL0 is set high and BL1 is set low,while setting the other BL and WLs at proper voltages as shown in FIG.14( a) to disable other program-1 and program-0 diodes. The bold line inFIG. 14( a) shows the direction of current flow. To program a 0 into thecell 310-01, WL0 is set low and BL1 is set high, while setting the otherBL and WLs at proper voltages as shown in FIG. 14( b) to disable theother program-1 and program-0 diodes. The bold line in FIG. 14( b) showsthe direction of current flow.

The embodiments of constructing MRAM cells in a 2×2 array as shown inFIGS. 12( a)-14(b) are for illustrative purposes. Those skilled in theart understand that the number of cells, rows, or columns in a memorycan be constructed arbitrarily and rows and columns are interchangeable.

The programmable resistive devices can be used to construct a memory inaccordance with one embodiment. FIG. 15 shows a portion of aprogrammable resistive memory 100 constructed by an array 101 of n-rowby (m+1)-column non-MRAM cells 110 and n wordline drivers 150-i, wherei=0, 1, . . . , n−1, in accordance with one embodiment. The memory array101 has m normal columns and one reference column for one shared senseamplifier 140 for differential sensing. Each of the memory cells 110 hasa resistive element 111 coupled to the P terminal of a diode 112 asprogram selector and to a bitline BLj 170-j (j=0, 1, . . . m−1) orreference bitline BLR0 175-0 for those of the memory cells 110 in thesame column. The N terminal of the diode 112 is coupled to a wordlineWLBi 152-i through a local wordline LWLBi 154-i, where i=0, 1, . . . ,n−1, for those of the memory cells 110 in the same row. Each wordlineWLBi is coupled to at least one local wordline LWLBi, where i=0, 1, . .. , n−1. The LWLBi 154-i is generally constructed by a high resistivitymaterial, such as N well or polysilicon, to connect cells, and thencoupled to the WLBi (e.g., a low-resistivity metal WLBi) throughconductive contacts or vias, buffers, or post-decoders 172-i, where i=0,1, . . . , n−1. Buffers or post-decoders 172-i may be needed when usingdiodes as program selectors because there are currents flowing throughthe WLBi, especially when one WLBi drives multiple cells for program orread simultaneously in other embodiments. The wordline WLBi is driven bythe wordline driver 150-i with a supply voltage vddi that can beswitched between different voltages for program and read. Each BLj 170-jor BLR0 175-0 is coupled to a supply voltage VDDP through a Y-write passgate 120-j or 125 for programming, where each BLj 170-j or BLR0 175-0 isselected by YSWBj (j=0, 1, . . . , m−1) or YSWRB0, respectively. TheY-write pass gate 120-j (j=0, 1, . . . , m−1) or 125 can be built byPMOS, though NMOS, diode, or bipolar devices can be employed in someembodiments. Each BL or BLR0 is coupled to a dataline DL or DLR0 througha Y-read pass gate 130-j or 135 selected by YSRj (j=0, 1, . . . , m−1)or YSRR0, respectively. In this portion of memory array 101, m normaldatalines DLj (j=0, 1, . . . , m−1) are connected to an input 160 of asense amplifier 140. The reference dataline DLR0 provides another input161 for the sense amplifier 140 (no multiplex is generally needed in thereference branch). The output of the sense amplifiers 140 is Q0.

To program a cell, the specific WLBi and YSWBj are turned on and a highvoltage is supplied to VDDP, where i=0, 1, . . . n−1 and j=0, 1, . . . ,m−1. In some embodiments, the reference cells can be programmed to 0 or1 by turning on WLRBi, and YSWRB0, where i=0, 1, . . . , n−1. To read acell, a data column 160 can be selected by turning on the specific WLBiand YSRj, where i=0, 1, . . . , n−1, and j=0, 1, . . . , m−1, and areference cell coupled to the reference dataline DLR0 161 for the senseamplifier 140 can be selected to sense and compare the resistancedifference between BLs and ground, while disabling all YSWBj and YSWRB0where j=0, 1, . . . , m−1.

The programmable resistive devices can be used to construct a memory inaccordance with one embodiment. FIG. 16( a) shows a portion of aprogrammable resistive memory 100 constructed by an array 101 of3-terminal MRAM cells 110 in n rows and m+1 columns and n pairs ofwordline drivers 150-i and 151-i, where i=0, 1, . . . , n−1, accordingto one embodiment. The memory array 101 has m normal columns and onereference column for one shared sense amplifier 140 for differentialsensing. Each of the memory cells 110 has a resistive element 111coupled to the P terminal of a program-0 diode 112 and N terminal of aprogram-1 diode 113. The program-0 diode 112 and the program-1 diode 113serve as program selectors. Each resistive element 111 is also coupledto a bitline BLj 1701 (j=0, 1, . . . m−1) or reference bitline BLR0175-0 for those of the memory cells 110 in the same column. The Nterminal of the diode 112 is coupled to a wordline WLNi 152-i through alocal wordline LWLNi 154-i, where i=0, 1, . . . , n−1, for those of thememory cells 110 in the same row. The P terminal of the diode 113 iscoupled to a wordline WLPi 153-i through a local wordline LWLPi 155-i,where i=0, 1, . . . , n−1, for those cells in the same row. Eachwordline WLNi or WLPi is coupled to at least one local wordline LWLNi orLWLPi, respectively, where i=0,1, . . . , n−1. The LWLNi 154-i and LWLPi155-i are generally constructed by a high resistivity material, such asN well or polysilicon, to connect cells, and then coupled to the WLNi orWLPi (e.g., low-resistivity metal WLNi or WLPi) through conductivecontacts or vias, buffers, or post-decoders 172-i or 173-i respectively,where i=0,1, . . . , n−1. Buffers or post-decoders 172-i or 173-i may beneeded when using diodes as program selectors because there are currentsflowing through WLNi or WLPi, especially when one WLNi or WLPi driversmultiple cells for program or read simultaneously in some embodiments.The wordlines WLNi and WLPi are driven by wordline drivers 150-i and151-i, respectively, with a supply voltage vddi that can be switchedbetween different voltages for program and read. Each BLj 170-j or BLR0175-0 is coupled to a supply voltage VDDP through a Y-write-0 pass gate120-0 or 125 to program 0, where each BLj 170-j or BLR0 175-0 isselected by YSOWBj (j=0, 1, . . . , m−1) or YS0WRB0, respectively.Y-write-0 pass gate 120-j or 125 can be built by PMOS, though NMOS,diode, or bipolar devices can be employed in other embodiments.Similarly, each BLj 170-j or BLR0 175-0 is coupled to a supply voltage0V through a Y-write-1 pass gate 121-j or 126 to program 1, where eachBLj 170-j or BLR0 175-0 is selected by YS1Wj (j=0, 1, . . . , m−1) orYS1WR0, respectively. Y-write-1 pass gate 121-j or 126 is can be builtby NMOS, though PMOS, diode, or bipolar devices can be employed in otherembodiments. Each BL or BLR0 is coupled to a dataline DL or DLR0 througha Y-read pass gate 130-j or 135 selected by YSRj (j=0, 1, . . . , m−1)or YSRR0, respectively. In this portion of memory array 101, m normaldatalines DLj (j=0, 1, . . . , m−1) are connected to an input 160 of asense amplifier 140. Reference dataline DLR0 provides another input 161for the sense amplifier 140, except that no multiplex is generallyneeded in a reference branch. The output of the sense amplifier 140 isQ0.

To program a 0 into a cell, the specific WLNi, WLPi and BLj are selectedas shown in FIG. 13( a) or 13(b) by wordline drivers 150-i, 151-i, andY-pass gate 120-j by YSOWBj, respectively, where i=0, 1, . . . n−1 andj=0, 1, . . . , m−1, while the other wordlines and bitlines are alsoproperly set. A high voltage is applied to VDDP. In some embodiments,the reference cells can be programmed into 0 by setting proper voltagesto WLRNi 158-i, WLRPi 159-i and YS0WRB0, where i=0,1, . . . , n−1. Toprogram a 1 to a cell, the specific WLNi, WLPi and BLj are selected asshown in FIG. 12( a) or 12(b) by wordline driver 150-i, 151-i, andY-pass gate 121-j by YS1Wj, respectively, where i=0, 1, . . . n−1 andj=0, 1, . . . , m−1, while the other wordlines and bitlines are alsoproperly set. In some embodiments, the reference cells can be programmedto 1 by setting proper voltages to WLRNi 158-i, WLRPi 159-i and YS1WR0,where i=0,1, . . . , n−1. To read a cell, a data column 160 can beselected by turning on the specific WLNi, WLPi and YSRj, where i=0, 1, .. . , n−1, and j=0, 1, . . . , m−1, and a reference cell coupled to thereference dataline DLR 161 for the sense amplifier 140 to sense andcompare the resistance difference between BLs and ground, whiledisabling all YS0WBj, YS0WRB0, YS1Wj and YS1WR0, where j=0, 1, . . . ,m−1.

Another embodiment of constructing an MRAM memory with 2-terminal MRAMcells is shown in FIG. 16( b), provided the voltage difference VDDP,between high and low states, is less than twice of the diode's thresholdvoltage Vd, i.e., VDDP<2*Vd. As shown in FIG. 16( b), two wordlines perrow WLNi 152-i and WLPi 153-i in FIG. 16( a) can be merged into onewordline driver WLNi 152-i, where i=0,1, . . . , n−1. Also, the localwordlines LWLNi 154-i and LWLP 155-i per row in FIG. 16( a) can bemerged into one local wordline LWLNi 154-i, where i=0, 1, . . . , n−1,as shown in FIG. 16( b). Still further, two wordline drivers 150-i and151-i in FIG. 16( a) can be merged into one, i.e., wordline driver150-i. The BLs and WLNs of the unselected cells are applied with properprogram 1 and 0 conditions as shown in FIGS. 14( a) and 14(b),respectively. Since half of wordlines, local wordlines, and wordlinedrivers can be eliminated in this embodiment, cell and macro areas canbe reduced substantially.

Differential sensing is a common for programmable resistive memory,though single-end sensing can be used in other embodiments. FIGS. 17(a), 17(b), and 17(c) show three other embodiments of constructingreference cells for differential sensing. In FIG. 17( a), a portion ofmemory 400 has a normal array 180 of n×m cells, two reference columns150-0 and 150-1 of n×1 cells each storing all data 0 and 1 respectively,m+1 Y-read pass gates 130, and a sense amplifier 140. As an example, n=8and m=8 are used to illustrate the concept. There are n wordlines WLBiand n reference wordlines WLRBi for each row, where i=0,1, . . . , n−1.When a wordline WLBi is turned on to access a row, a correspondingreference wordline WLRBi (i=0, 1, . . . , n−1) is also turned on toactivate two reference cells 170-0 and 170-1 in the same row to providemid-level resistance after proper scaling in the sense amplifier. Theselected dataline 160 along with the reference dataline 161 are input toa sense amplifier 140 to generate an output Q0. In this embodiment, eachWLRBi and WLBi (i=0, 1, . . . , n−1) are hardwired together and everycells in the reference columns need to be pre-programmed before read.

FIG. 17( b) shows another embodiment of using a reference cell externalto a reference column. In FIG. 17( b), a portion of memory 400 has anormal array 180 of n×m cells, a reference column 150 of n×1 cells, m+1Y-read pass gates 130, and a sense amplifier 140. When a wordline WLBi(i=0, 1, . . . , n−1) is turned on, none of the cells in the referencecolumn 150 are turned on. An external reference cell 170 with apredetermined resistance is turned on instead by an external referencewordline WLRB. The selected dataline 160 and the reference dataline 161are input to a sense amplifier 140 to generate an output Q0. In thisembodiment, all internal reference wordlines WLRBi (i=0, 1, . . . , n−1)in each row are tied together to a high voltage to disable the diodes inthe reference column. The reference column 150 provides a loading tomatch with that of the normal columns.

FIG. 17( c) shows another embodiment of constructing reference cells fordifferential sensing. In FIG. 17( c), a portion of memory 400 has anormal array 180 of n×m cells, one reference column 150 of n×1, tworeference rows 175-0 and 175-1 of 1×m cells, m+1 Y-read pass gates 130,and a sense amplifier 140. As an example, n=8 and m=8 are used toillustrate the approach. There are n wordlines WLBi and 2 referencewordlines WLRB0 175-0 and WLRB1 175-1 on top and bottom of the array,where i=0, 1, . . . , n−1. When a wordline WLBi (i=0, 1, . . . , n−1) isturned on to access a row, the reference wordline WLRB0 and WLRB1 arealso turned on to activate two reference cells 170-0 and 170-1 in theupper and lower right corners of the array 180, which store data 0 and 1respectively. The selected dataline 160 along with the referencedataline 161 are input to a sense amplifier 140 to generate an outputQ0. In this embodiment, all cells in the reference column 150 aredisabled except that the cells 170-0 and 170-1 on top and bottom of thereference column 150. Only two reference cells are used for the entiren×m array that needs to be pre-programmed before read.

For those programmable resistive devices that have a very smallresistance ratio between states 1 and 0, such as 2:1 ratio in MRAM,FIGS. 17( a) and 17(c) are desirable embodiments, depending on how manycells are suitable for one pair of reference cells. Otherwise, FIG. 17(b) is a desirable embodiment for electrical fuse or PCM that hasresistance ratio of more than about 10.

FIGS. 15, 16(a), 16(b), 17(a), 17(b), and 17(c) show only a fewembodiments of a portion of programmable resistive memory in asimplified manner. The memory array 101 in FIGS. 15, 16(a), and 16(b)can be replicated s times to read or program s-cells at the same time.In the case of differential sensing, the number of reference columns tonormal columns may vary and the physical location can also vary relativeto the normal data columns. Rows and columns are interchangeable. Thenumbers of rows, columns, or cells likewise may vary. For those skilledin the art understand that the above descriptions are for illustrativepurpose. Various embodiments of array structures, configurations, andcircuits are possible and are still within the scope of this invention.

The portions of programmable resistive memories shown in FIGS. 15,16(a), 16(b), 17(a), 17(b) and 17(c) can include different types ofresistive elements. The resistive element can be an electrical fuseincluding a fuse fabricated from an interconnect, contact/via fuse,contact/via anti-fuse, or gate oxide breakdown anti-fuse. Theinterconnect fuse can be formed from silicide, metal, metal alloy, orsome combination thereof, or can be constructed from a CMOS gate. Theresistive element can also be fabricated from phase-change material,MTJ, etc. For the electrical fuse fabricated from an interconnect,contact, or via fuse, programming requirement is to provide asufficiently high current, about 4-20 mA range, for a few microsecondsto blow the fuse by electro-migration, heat, ion diffusion, or somecombination thereof. For anti-fuse, programming requirement is toprovide a sufficiently high voltage to breakdown the dielectrics betweentwo ends of a contact, via or CMOS gate. The required voltage is about6-7V for a few millisecond to consume about 10 uA of current in today'stechnologies. Programming Phase-Change Memory (PCM) requires differentvoltages and durations for 0 and 1. Programming to a 1 (or to reset)requires a high and short voltage pulse applied to the phase-changefilm. Alternatively, programming to a 0 (or to set) requires a low andlong voltage pulse applied to the phase change film. The reset needsabout 3V for 50 ns and consumes about 300 uA, while set needs about 2Vfor 300 ns and consumes about 100 uA. For MRAM, the high and low programvoltages are about 2-3V and 0V, respectively, and the current is about+/−100-200 uA.

Most programmable resistive devices have a higher voltage VDDP (˜2-3V)for programming than the core logic supply voltage VDD (˜1.0V) forreading. FIG. 18( a) shows a schematic of a wordline driver circuit 60according to one embodiment. The wordline driver includes devices 62 and61, as shown as the wordline driver 150 in FIGS. 15, 16(a) and 16(b).The supply voltage vddi is further coupled to either VDDP or VDD throughpower selectors 63 and 64 (e.g., PMOS power selectors) respectively. Theinput of the wordline driver Vin is from an output of an X-decoder. Insome embodiments, the power selectors 63 and 64 are implemented as thickoxide I/O devices to sustain high voltage. The bodies of power selector63 and 64 can be tied to vddi to prevent latchup.

Similarly, bitlines tend to have a higher voltage VDDP (˜2-3V) forprogramming than the core logic supply voltage VDD (˜1.0V) for reading.FIG. 18( b) shows a schematic of a bitline circuit 70 according to oneembodiment. The bitline circuit 70 includes a bitline (BL) coupled toVDDP and VDD through power selectors 73 and 74 (e.g., PMOS powerselectors), respectively. If the bitline needs to sink a current such asin an MRAM, an NMOS pulldown device 71 can be provided. In someembodiments, the power selectors 73 and 74 as well as the pulldowndevice 71 can be implemented as thick-oxide I/O devices to sustain highvoltage. The bodies of power selector 73 and 74 can be tied to vddi toprevent latchup.

Using diodes as program selectors may have high leakage current if amemory size is very large. Power selectors for a memory can helpreducing leakage current by switching to a lower supply voltage or eventurning off when a portion of memory is not in use. FIG. 18( c) shows aportion of memory 85 with an internal power supply VDDP coupled to anexternal supply VDDPP and a core logic supply VDD through powerselectors 83 and 84. VDDP can even be coupled to ground by an NMOSpulldown device 81 to disable this portion of memory 85, if this portionof memory is temporarily not in use.

FIGS. 19( a) and 20(a) only show two of many pre-amplifier embodiments.Similarly, FIGS. 19( b), 20(b) and 20(c) only show several of manyamplifier and level shifter embodiments. Various combinations ofpre-amplifiers, level shifters, and amplifiers in core logic or I/Odevices can be constructed differently, separately or mixed.

FIG. 19( a) shows one embodiment of a schematic of a pre-amplifier 100according to one embodiment. The pre-amplifier 100 needs specialconsiderations because the supply voltage VDD for core logic devices isabout 1.0V that does not have enough head room to turn on a diode tomake sense amplifiers functional, considering a diode's threshold isabout 0.7V. One embodiment is to use another supply VDDR, higher thanVDD, to power at least the first stage of sense amplifiers. Theprogrammable resistive cell 110 shown in FIG. 19( a) has a resistiveelement 111 and a diode 112 as program selector, and can be selected forread by asserting YSR′ to turn on a gate of a NMOS 130 (NMOS device) andwordline bar WLB. The pre-amplifier 100 also has a reference cell 115including a reference resistive element 116 and a reference diode 117.The reference cell 115 can be selected for differential sensing byasserting YSR′ to turn on a gate of a NMOS 131 and reference wordlineWLRB. The resistance Ref of the reference resistive element 116 can beset at a resistance half-way between minimum of state 1 and maximum ofstate 0 resistance.

The drains of NMOS 130 and 131 are coupled to sources of NMOS 132 and134, respectively. The gates of 132 and 134 are biased at a fixedvoltage Vbias. The channel width to length ratios of NMOS 132 and 134can be relatively large to clamp the voltage swings of bitline BL andreference bitline BLR, respectively. The drain of NMOS 132 and 134 arecoupled to drains of PMOS 170 and 171, respectively. The drain of PMOS170 is coupled to the gate of PMOS 171 and the drain of PMOS 171 iscoupled to the gate of PMOS 170. The outputs V+ and V− of thepre-amplifier 100 are drains of PMOS 170 and PMOS 171 respectively. Thesources of PMOS 170 and PMOS 171 are coupled to a read supply voltageVDDR. The outputs V+ and V− are pulled up by a pair of PMOS 175 to VDDRwhen the pre-amplifier 100 is disabled. VDDR is about 2-3V (which ishigher than about 1.0V VDD of core logic devices) to turn on the diodeselectors 112 and 117 in the programmable resistive cell 110 and thereference cell 115, respectively. The CMOS 130, 131, 132, 134, 170, 171,and 175 can be embodied as thick-oxide I/O devices to sustain highvoltage VDDR. In another embodiment, the read selectors 130 and 131 canbe PMOS devices.

FIG. 19( b) shows one embodiment of a schematic of an amplifier 200according to one embodiment. In another embodiment, the outputs V+ andV− of the pre-amplifier 100 in FIG. 19( a) can be coupled to gates ofNMOS 234 and 232, respectively, of the amplifier 200. The NMOS 234 and232 can be relatively thick oxide I/O devices to sustain the high inputvoltage V+ and V− from a pre-amplifier. The sources of NMOS 234 and 232are coupled to drains of NMOS 231 and 230, respectively. The sources ofNMOS 231 and 230 are coupled to a drain of an NMOS 211. The gate of NMOS211 is coupled to a clock φ to turn on the amplifier 200, while thesource of NMOS 211 is coupled to ground. The drains of NMOS 234 and 232are coupled to drains of PMOS 271 and 270, respectively. The sources ofPMOS 271 and 270 are coupled to a core logic supply VDD. The gates ofPMOS 271 and NMOS 231 are connected and coupled to the drain of PMOS270, as a node Vp. Similarly, the gates of PMOS 270 and NMOS 230 areconnected and coupled to the drain of PMOS 271, as a node Vn. The nodesVp and Vn are pulled up by a pair of PMOS 275 to VDD when the amplifier200 is disabled when φ goes low. The output nodes Vout+ and Vout− arecoupled to nodes Vn and Vp through a pair of inverters as buffers.

FIG. 19( c) shows a timing diagram of the pre-amplifier 100 and theamplifier 200 in FIGS. 19( a) and 19(b), respectively. The X- andY-addresses AX/AY are selected to read a cell. After some propagationdelays, a cell is selected for read by turning WLB low and YSR high tothereby select a row and a column, respectively. Before activating thepre-amplifier 100, a pulse Vpc is generated to precharge DL and DLR toground. The pre-amplifier 100 would be very slow if the DL and DLRvoltages are high enough to turn off the cascode devices (e.g., NMOS 132and 134). After the pre-amplifier outputs V+ and V− are stabilized, theclock φ is set high to turn on the amplifier 200 and to amplify thefinal output Vout+ and Vout− into full logic levels.

FIG. 20( a) shows another embodiment of a pre-amplifier 100′, similar tothe pre-amplifier 100 in FIG. 18( a). The reference branch is turned onby a level signal to enable a sense amplifier, SAEN, rather than cycleby cycle in FIG. 19( a). The PMOS pull-ups 171 and 170 in FIG. 20( a)are configured as current mirror loads, rather than a pair ofcross-coupled PMOS in FIG. 19( a). In this embodiment, the number of thereference branches can be shared at the expense of increasing powerconsumption.

FIG. 20( b) shows level shifters 300 according to one embodiment. The V+and V− from the pre-amplifier 100, 100′ outputs in FIG. 19( a) or FIG.20( a) are coupled to gates of NMOS 301 and 302, respectively. Thedrains of NMOS 301 and 302 are coupled to a supply voltage VDDR. Thesources of NMOS 301 and 302 are coupled to drains of NMOS 303 and 304,respectively, which have gates and drains connected as diodes to shiftthe voltage level down by one Vtn, the threshold voltage of an NMOS. Thesources of NMOS 303 and 304 are coupled to pulldown devices NMOS 305 and306, respectively. The gates of NMOS 305 and 306 can be turned on by aclock φ. The NMOS 301, 302, 303 and 304 can be thick-oxide I/O devicesto sustain high voltage VDDR. The NMOS 303 and 304 can be cascaded morethan once to shift V+ and V− further to proper voltage levels Vp and Vn.In another embodiment, the level shifting devices 303 and 304 can bebuilt using PMOS devices.

FIG. 20( c) shows another embodiment of an amplifier 200′ withcurrent-mirror loads having PMOS 270 and 271. The inputs Vp and Vn ofthe amplifier 200′ are from the outputs Vp and Vn of the level shifter300 in FIG. 20( b) can be coupled to gates of NMOS 231 and 230,respectively. The drains of NMOS 231 and 230 are coupled to drains ofNMOS 271 and 270 which provide current-mirror loads. The drain and gateof PMOS 271 are connected and coupled to the gate of PMOS 270. Thesources of NMOS 231 and 230 are coupled to the drain of an NMOS 211,which has the gate coupled to a clock signal φ and the source to ground.The clock signal φ enables the amplifier 200. The drain of PMOS 270provides an output Vout+. The PMOS pullup 275 keeps the output Vout+ atlogic high level when the amplifier 200′ is disabled.

FIGS. 21( a) and 21(b) show a flow chart depicting embodiments of aprogram method 700 and a read method 800, respectively, for aprogrammable resistive memory in accordance with certain embodiments.The methods 700 and 800 are described in the context a programmableresistive memory, such as the programmable resistive memory 100 in FIGS.15, 16(a) and 16(c). In addition, although described as a flow of steps,one of ordinary skilled in the art will recognize that at least some ofthe steps may be performed in a different order, includingsimultaneously, or skipped.

FIG. 21( a) depicts a method 700 of programming a programmable resistivememory in a flow chart according to one embodiment. In the first step710, proper power selectors can be selected so that high voltages can beapplied to the power supplies of wordline drivers and bitlines. In thesecond step 720, the data to be programmed in a control logic (not shownin FIGS. 15, 16(a), and 16(b)) can be analyzed, depending on what typesof programmable resistive devices. For electrical fuse, this is aOne-Time-Programmable (OTP) device such that programming always meansblowing fuses into a non-virgin state and is irreversible. Programvoltage and duration tend to be determined by external control signals,rather than generated internally from the memory. For PCM, programminginto a 1 (to reset) and programming into a 0 (to set) require differentvoltages and durations such that a control logic determines the inputdata and select proper power selectors and assert control signals withproper timings. For MRAM, the directions of current flowing through MTJsare more important than time duration. A control logic determines properpower selectors for wordlines and bitlines and assert control signals toensure a current flowing in the desired direction for desired time. Inthe third step 730, a cell in a row can be selected and thecorresponding local wordline can be turned on. In the fourth step 740,sense amplifiers can be disabled to save power and prevent interferencewith the program operations. In the fifth step 750, a cell in a columncan be selected and the corresponding Y-write pass gate can be turned onto couple the selected bitline to a supply voltage. In the last step760, a desired current can be driven for a desired time in anestablished conduction path to complete the program operations. For mostprogrammable resistive memories, this conduction path is from a highvoltage supply through a bitline select, resistive element, diode asprogram selector, and an NMOS pulldown of a local wordline driver toground. Particularly, for programming a 1 to an MRAM, the conductionpath is from a high voltage supply through a PMOS pullup of a localwordline driver, diode as program selector, resistive element, andbitline select to ground.

FIG. 21( b) depicts a method 800 of reading a programmable resistivememory in a flow chart according to one embodiment. In the first step810, proper power selectors can be selected to provide supply voltagesfor local wordline drivers, sense amplifiers, and other circuits. In thesecond step 820, all Y-write pass gates, i.e. bitline program selectors,can be disabled. In the third step 830, desired local wordline(s) can beselected so that the diode(s) as program selector(s) have a conductionpath to ground. In the fourth step 840, sense amplifiers can be enabledand prepared for sensing incoming signals. In the fifth step 850, thedataline and the reference dataline can be pre-charged to the V− voltageof the programmable resistive device cell. In the sixth step 860, thedesired Y-read pass gate can be selected so that the desired bitline iscoupled to an input of the sense amplifier. A conduction path is thusestablished from the bitline to the resistive element in the desiredcell, diode(s) as program selector(s), and the pulldown of the localwordline driver(s) to ground. The same applies for the reference branch.In the last step 870, the sense amplifiers can compare the read currentwith the reference current to determine a logic output of 0 or 1 tocomplete the read operations.

FIG. 22 shows a processor system 700 according to one embodiment. Theprocessor system 700 can include a programmable resistive device 744,such as in a cell array 742, in memory 740, according to one embodiment.The processor system 700 can, for example, pertain to a computer system.The computer system can include a Central Process Unit (CPU) 710, whichcommunicate through a common bus 715 to various memory and peripheraldevices such as I/O 720, hard disk drive 730, CDROM 750, memory 740, andother memory 760. Other memory 760 is a conventional memory such asSRAM, DRAM, or flash, typically interfaces to CPU 710 through a memorycontroller. CPU 710 generally is a microprocessor, a digital signalprocessor, or other programmable digital logic devices. Memory 740 ispreferably constructed as an integrated circuit, which includes thememory array 742 having at least one programmable resistive device 744.The memory 740 typically interfaces to CPU 710 through a memorycontroller. If desired, the memory 740 may be combined with theprocessor, for example CPU 710, in a single integrated circuit.

Embodiments of the invention can be implemented in a part or all of anintegrated circuit in a Printed Circuit Board (PCB), or in a system. Theprogrammable resistive device can be fuse, anti-fuse, or emergingnonvolatile memory. The fuse can be silicided or non-silicidedpolysilicon fuse, metal fuse, contact fuse, or via fuse. The anti-fusecan be a gate-oxide breakdown anti-fuse, contact or via anti-fuse withdielectrics in-between. The emerging nonvolatile memory can be MagneticRAM (MRAM), Phase Change Memory (PCM), Conductive Bridge RAM (CBRAM), orResistive RAM (RRAM). Though the program mechanisms are different, theirlogic states can be distinguished by different resistance values.

The above description and drawings are only to be consideredillustrative of exemplary embodiments, which achieve the features andadvantages of the present invention. Modifications and substitutions ofspecific process conditions and structures can be made without departingfrom the spirit and scope of the present invention.

The many features and advantages of the present invention are apparentfrom the written description and, thus, it is intended by the appendedclaims to cover all such features and advantages of the invention.Further, since numerous modifications and changes will readily occur tothose skilled in the art, it is not desired to limit the invention tothe exact construction and operation as illustrated and described.Hence, all suitable modifications and equivalents may be resorted to asfalling within the scope of the invention.

What is claimed is:
 1. A reversible resistive memory, comprising: aplurality of reversible resistive cells, each of the reversibleresistive cells including a reversible resistive film and a diode, thediode serves as a program selector and is constructed from a polysiliconstructure having a first terminal from a first region with a first typeof dopant and a second terminal from a second region with a second typeof dopant, the first terminal of the diode coupled to a first terminalof the reversible resistive film, the reversible resistive film beingprogrammable reversibly by conducting a current flowing through thereversible resistive film, wherein the current is high or short durationfor programming one state and low or long duration for programminganother state; a plurality of local wordlines, each coupled to aplurality of the reversible resistive cells via the second terminal ofthe diodes and having a first resistivity; a plurality of globalwordlines, each coupled to at least one of the local wordlines andhaving a second resistivity; and a plurality of bitlines, each coupledto a plurality of the reversible resistive cells via the second terminalof reversible resistive film.
 2. A reversible resistive memory asrecited in claim 1, wherein the reversible resistive film is aphase-change film that has chemical composition of Germanium (Ge),Antimony (Sb), and Tellurium (Te).
 3. A reversible resistive memory asrecited in claim 1, wherein the reversible resistive film is aphase-change film that has chemical composition of Germanium (Ge),Antimony (Sb), and Tellurium (Te) as Ge_(x)Sb_(y)Te_(z), where x+y+z=9,and x=2+/−10%, y=2+/−10%, and z=5+−10%.
 4. A reversible resistive memoryas recited in claim 1, wherein the reversible resistive film comprises ametal-oxide film between metal or metal alloy electrodes.
 5. Areversible resistive memory as recited in claim 1, wherein thereversible resistive film comprises solid-state electrolyte film betweenmetal or metal alloy electrodes.
 6. A reversible resistive memory asrecited in claim 1, wherein the local wordline is constructed at leastin part of polysilicon.
 7. A reversible resistive memory as recited inclaim 1, wherein the global wordline is constructed of metal.
 8. Areversible resistive memory as recited in claim 1, wherein the globalwordline is coupled to the local wordline through at least one ofconductive via or contact.
 9. A reversible resistive memory as recitedin claim 1, wherein the bitline is coupled to a first supply voltageline and the global wordline is coupled to a second supply voltage lineduring program or read, and wherein the voltage between the first andsecond supply voltage lines has a larger magnitude or longer durationfor program than for read.
 10. A reversible resistive memory as recitedin claim 1, wherein the bitline is coupled to a first supply voltageline and the global wordline is coupled to a second supply voltage lineduring program, and wherein the voltage between the first and secondsupply voltages lines has a larger magnitude or longer duration forprogramming into the one state than the another state.
 11. A reversibleresistive memory as recited in claim 1, wherein the bitline is coupledto a first supply voltage line and the global wordline is coupled to asecond supply voltage line during program, and wherein the voltagebetween the first and second supply voltage lines has voltage-limit orcurrent-limit to program into one state.
 12. A reversible resistivememory as recited in claim 1, wherein the reversible resistive memoryfurther comprises a plurality of write bitline selectors to select abitline coupled to a first supply voltage line, and a plurality ofwordline drivers to select a local wordline coupled to a second supplyvoltage line to conduct a current during program.
 13. A reversibleresistive memory as recited in claim 1, wherein the reversible resistivememory further comprising a plurality of read bitline selectors toselect a bitline coupled to a sense amplifier, and a plurality ofwordline drivers to select a local wordline coupled to a supply voltageline to conduct a current through the sense amplifier during read. 14.An electronic system, comprising: a processor; and a reversibleresistive memory operatively connected to the processor, the reversibleresistive memory including at least: a plurality of reversible resistivecells, each of the reversible resistive cells including a reversibleresistive film and a diode, the diode serves as a program selector andis constructed from a polysilicon structure having a first terminal froma first region with a first type of dopant and a second terminal from asecond region with a second type of dopant, the first terminal of thediode coupled to a first terminal of the reversible resistive film, thereversible resistive film being programmable reversibly by conducting acurrent flowing through the reversible resistive film, wherein thecurrent is high or short duration for programming one state and low orlong duration for programming another state; a plurality of localwordlines, each coupled to a plurality of the reversible resistive cellsvia the second terminal of the diodes and having a first resistivity; aplurality of global wordlines, each coupled to at least one of the localwordlines and having a second resistivity; and a plurality of bitlines,each coupled to a plurality of the reversible resistive cells via thesecond terminal of reversible resistive film.
 15. A reversible resistivecell as recited in claim 14, wherein the reversible resistive filmcomprises a phase-change film that has chemical composition includingone or more of Germanium (Ge), Antimony (Sb), and Tellurium (Te).
 16. Areversible resistive cell as recited in claim 14, wherein the reversibleresistive film comprises a metal-oxide film between metal or metal alloyelectrodes.
 17. A reversible resistive cell as recited in claim 14,wherein the reversible resistive film comprises solid-state electrolytefilm between metal or metal alloy electrodes.
 18. A reversible resistivememory as recited in claim 14, wherein the local wordline is constructedat least in part of polysilicon.
 19. An electronic system as recited inclaim 14, wherein the electronic system is configured to periodicallyread content of the memory cells and write the content back.
 20. Amethod for programming a reversible resistive memory, comprising:providing a plurality of reversible resistive cells, each of thereversible resistive memory cells including a reversible resistive filmand a diode, the diode serves as program selector and is constructedfrom a polysilicon structure having a first terminal from a first regionwith a first type of dopant and a second terminal from a second regionwith a second type of dopant, the first terminal of the diode beingcoupled to a first terminal of the reversible resistive film, thereversible resistive film being programmable reversibly by conducting acurrent flowing through the reversible resistive film; providing aplurality of local wordlines, each coupled to a plurality of thereversible resistive cells via the second terminal of the diodes andhaving a first resistivity; providing a plurality of global wordlines,each coupled to at least one of the local wordlines and having a secondresistivity; providing a plurality of bitlines, each coupled to aplurality of the reversible resistive cells via a second terminal ofreversible resistive film; and programming at least one of thereversible resistive cells by applying voltages to a selected one of theglobal wordlines and to a selected one of the bitlines, and generating ahigh or short-duration current to reversibly program into one state anda low or long-duration current to reversibly program into another state.